Merge pull request #16247 from fabi1cazenave/smsUpdateThread-bug968815

Bug 968815 - [SMS] update thread when last message is deleted, r=julienw
(cherry picked from commit 988c320bccfa7f873dc2ef3e7c34a11a9f4ad409)
Merge pull request #16421 from fabi1cazenave/smsUpdateThread-bug968815-v1.3

Bug 968815, uplift to v1.3